Rain changed over to snow in the wake of a powerful cold front that brought driving rain and strong winds to Rochester early Friday.

The sudden drop in temperatures caused precipitation to freeze on road surfaces early Friday, forming patches of what is known as black ice — a thin, all-but-invisible layer of ice that makes driving treacherous.

On Thursday evening, the weather service issued a lakeshore flood warning, citing strong winds, building waves and above-normal lake levels. At the shoreline, the wind-driven waves could cause significant erosion and minor flooding. The warning is in effect through 6 p.m. Friday.

The area will have to endure a genuinely wintry Friday: a high temperature of just 27 degrees, strong northwest winds and some snow showers.

The overnight low Friday into Saturday morning will be 18 degrees in metro Rochester and colder in the hills to the south. Wind chills will approach zero degrees.

On average, the first short-lived flakes are spotted in Rochester on Oct. 23, according to weather service climate records.

Our first measurable snowfall occurs, on average, on Nov. 8. Measurable snow is defined as one-tenth of an inch or more.